The present invention relates generally to a device and method for accurately measuring and dispensing a volume of a liquid by eliminating or minimizing the effects liquid density and liquid vapor pressure have on volumetric measurement. This eliminates the need to recalibrate the measuring device for use with a different liquid.
In chemistry labs and other research and industrial labs where basic chemistry methods and procedures are employed, it is frequently necessary to measure the volume of various liquids. Certain syringe and pump type devices have been used for volumetric measurement, but each has its limitations. Examples are the pipette or pipettor, the syringe pump, and the bottletop dispenser.
The pipette to which we refer is a hand operated syringe having a spring loaded plunger, a disposable tip, and a means for repeatably controlling the travel of the plunger within the cylinder such as a movable stop. The pipette is operated by: depressing the plunger to the stop; inserting the tip into the liquid to be aspirated, measured and dispensed; removing the tip from the container from which the liquid was aspirated; placing the pipette tip over the container into which the liquid is to be dispensed; and depressing the plunger, thus expelling the liquid into the container receiving the liquid. In traditional pipettes, the aspirated liquid is wholly contained in pipette tip and liquid never contacts plunger. A consequence is the existence of a gas volume between the liquid and the plunger.
Most pipettes are calibrated with distilled water at a standard temperature. When aspirating a liquid having properties which differ from water, the volume aspirated at any given stop position can differ significantly from the volume of water which would be aspirated at the same stop position. The specific liquid properties affecting accuracy are density and vapor pressure. Recalibration for the second liquid is only valid for the stop position at which the recalibration was conducted. The range of the scale indicating volume of liquid to be aspirated as a function of stop position will be different for the second liquid. Stated differently, for given minimum and maximum volumes of aspirated liquid, the distance between the minimum and maximum stop locations for the second liquid will be different than those for water. Complicating the matter of accuracy even further, the volume of liquid aspirated can be highly dependent on temperature owing to slight variations in density and much larger variations in vapor pressure.
Recently, pipette manufacturers have recognized a way to overcome the effects of density and vapor pressure on accuracy. Syringe tips for pipettes are now commercially available. The syringe tip has the appearance of common syringe, but has an elongated integral plastic tip having a diameter which is reduced with respect to the syringe body. What differentiates the syringe tip from the traditional syringe is the plunger. The plunger is designed so that when it is fully advanced, it occupies all free volume within the syringe tip as well as isolating free volume from the liquid in the transition zone between the syringe body and syringe tip. By eliminating gas from the syringe, the effects of liquid density and vapor pressure are largely negated.
The pipette with syringe tip is well suited to manual volumetric dispensing in which the user is relied upon to position the syringe tip for aspiration and dispensing of liquid. However, this concept does not work when tubing or valves are added to automate the liquid measurement and dispensing process. A solid syringe plunger tip cannot be made to penetrate a curved fluid conduit nor fill the complex internal geometry of a valve. Thus, this technology cannot be used to exclude all free volume when fluid aspiration involves the use of curved fluid conduits or valves.
Others have attempted to overcome this limitation by using a pusher fluid to occupy free volume within liquid volumetric and dispensing systems. Typically, the pusher fluid is either “immiscible” with the liquid to be aspirated or separated from the aspirated liquid by a bubble. To aspirate liquid, the pusher fluid is advanced to the orifice through which aspiration is to occur, the orifice is immersed in the liquid to be aspirated, and the pusher fluid is retracted from the orifice. Except in situations where liquid volumes and tubing diameters are extremely small, this approach does not work well. If a bubble is used to maintain separation between the aspirated liquid and the pusher fluid, the diameter of the containment for the aspirated fluid must be kept small or the bubble will collapse and the aspirated liquid and pusher fluid will commingle causing both accuracy and cross contamination problems. If the separating bubble is eliminated, the immiscible fluid will contact the aspirated liquid. When dispensing, it is difficult if not impossible to accurately detect the interface between the two different liquids so that all of the aspirated liquid but none of the pusher fluid is dispensed. In addition, cross contamination is a concern since no one pusher fluid is likely to be immiscible with all liquids which might be aspirated.
Another example of prior art is the syringe pump. The syringe pump is a syringe, the plunger of which is equipped with means such as a lead screw and stepper motor to move the plunger in a predictable and repeatable fashion. In any practical application, the syringe pump must be configured with tubing and valves to automate the liquid aspiration and expulsion processes. Upon the first aspiration of any given liquid, gas, owing to free volume within tubing and valves, will be aspirated into the syringe. As with the pipette, accurate volumetric measurement via the syringe pump depends on the elimination of gas from the system. If gas is allowed to remain in the system, the volume aspirated will depend on the density and vapor pressure of the liquid. After purging the gas, repetitive dispensing can be accomplished with acceptable accuracy. However, if aspiration of a second liquid is desired, it is necessary to purge the syringe pump of the first liquid to avoid cross contamination. The second liquid may then be aspirated, but it will again be necessary to purge gas from the system. Alternatively, one may use multiple syringe pumps, each dedicated to a single liquid, but this practice becomes expensive.
The bottletop dispenser is yet another example in which a single liquid is repetitively measured and dispensed. This device also needs to be purged of any gas prior to use to insure volumetric accuracy. The simplest way to accomplish this bleeding operation is to dispense liquid until gas is eliminated from the device. This results in waste liquid which must be either disposed of or returned to the bottle at some later time. Recognizing this limitation, a valve and conduit have been incorporated into some bottletop dispensers so that liquid can either be returned to the bottle or dispensed depending on the position of the valve. Nevertheless, the bottletop dispenser shares a major shortcoming with the syringe pump. If one desires to dispense a second liquid with the bottletop dispenser, this device must be purged of the first liquid, cleaned to avoid cross contamination, and primed with the second liquid. Thus it is more practical to devote a separate bottletop dispenser to each liquid to be dispensed, but this practice is also expensive.
The limitations associated with the devices discussed above illustrate the need for an accurate liquid volumetric measurement and dispensing device which compensates for the effects of liquid density and vapor pressure, which can measure and dispense unique dissimilar liquids in succession without the need for intermediate cleaning or bleeding operations, which can aspirate and dispense liquid through conduits and valves without a loss of accuracy, and which can easily be adapted to automated operation.
Accordingly, several objects and advantages of the present invention are:
(a) the accurate dispensing of a variety of liquids regardless of vapor pressure or density;
(b) the ability to measure and dispense dissimilar liquids in succession without the need to perform a separate operation to purge the prior liquid from the system;
(c) the ability to remove free volume from the volumetric measurement system without removing aspirated liquid from the volumetric measurement system;
(d) removal of free volume from components having complex geometry such as curved conduits and valves;
(e) its adaptability to automated operation.
Further objects and advantages of our invention will become apparent from a consideration of the drawings and ensuing description.